It is common practice to add chemicals to sewage in order to disinfect same prior to discharge thereof into the surrounding watershed. Recently, considerable attention has been directed to the on-site generation of such disinfecting chemicals, especially sodium hypochlorite, in electrolytic cells. Such on-site production is desirable owing to the problems and expense involved in shipping and storing dilute hypochlorite solutions and the dangers in handling the more concentrated chlorine gas in urban areas. Such processes are particularly attractive in coastal locations where the salt values of sea water provide an economical source of the hypochlorite precursor, sodium chloride. Inland, electrolytes may be prepared from solid sodium chloride, whether pure or impure.
The electrolysis of sea, brackish, or impure salt water leads to a pronounced problem, however, in the nature of the formation of heavy deposits on the cathodic surfaces. These deposits, generally of calcium and magnesium hydroxides and carbonates, are apparently caused by the hardness (calcium, magnesium) values in the electrolyte and lead to pronounced increases in operating cell voltage and ultimately build up to such a thickness as to impede or prevent electrolyte flow.
Therefore, considerable attention has been directed to the removal, prevention, or reduction of these deposits. Among the proposed solutions has been the use of air blasts or acid washes to remove the deposits. Reduction or prevention methods proposed include current density control, the use of streamline electrolyte flow, and high electrolyte velocity. None of these have resulted in a commercially feasible technique because of the down-time which is still required for cleaning and/or the reduction in operating efficiency which seems inevitably to follow, all of which increase the cost of operation.